The refractive index of optical waveguides can be increased by exposing the waveguide to a light source. Typically, these devices are produced by 1) irradiating the fibers from the side with ultraviolet or infrared radiation, or 2) in some devices, launching visible light to the fiber core in a predetermined manner. These photo-induced devices (referred to as gratings hereafter) help create a structure similar to a filter for multiplexing and demultiplexing different wavelengths and controlling wave propagation through the waveguide. The refractive index changes can be increased even more by "loading" the fibers with molecular hydrogen.
Typically, these gratings are a periodic structure of refractive index changes within the fiber or waveguide and are preferably formed by exposing one side of the optical waveguide to a light source so that a small section has a refractive index rise. For example, a coherent light beam from an ultraviolet source could be split into two parts and then recombined to form an interference pattern. The interference pattern with fringes of light and dark spots can be impinged upon the waveguide to form the desired grating.
It has been observed that the side-exposure of fiber with a light to form the index changes contributed to anisotropy in the waveguide. Until the present invention, it was not known what contributed to anisotropy. Some researchers proposed that stress contributed to the anisotropy since birefringence decreases upon heating or prolonged ultraviolet exposure. With earlier designed optical waveguide communication systems, any birefringence such as caused by writing the photo-induced gratings was at most minimal to the overall operation. However, technological advances in telecommunications and signal propagation will make the birefringence effect unacceptable. Anisotropy can cause dispersion within the fiber and small changes in wavelength propagation of just a few hundreds of a nanometer. In future, more advanced systems, these slight changes could correspond to a different communication channel in a multiplexed, multichannel system.
A critical issue in evaluating waveguide performance is the ability to investigate the light guiding structure within the waveguide, i.e., the physical, chemical, and optical characteristics of the core region where most of the light travels. Therefore, it would also be desirable to study the waveguide's endface (cross-section) to investigate and determine changes in the refractive index of the optical waveguide to further enhance research, development, and manufacturing of waveguides for future use.